The long-term consequences of methamphetamine (METH) abuse include a persistent, partial loss of monoamine systems in the brain, particularly the dopamine (DA) innervation of the striatum. Several events post-synaptic to DA terminals and occurring shortly (within ~ 8 hr) after administration of a neurotoxic regimen of METH are implicated in this neurotoxicity. However, the nature of the relationship between these post-synaptic events and the neurotoxic consequences of that treatment on the DA nerve terminal is less clear. One factor that has been implicated in this neurotoxicity is the production of nitric oxide (NO). Although it is relatively clear that NO contributes to METH-induced neurotoxicity to the DA nerve terminal, the source of this NO has not been clearly delineated. Evidence suggests that the generation of NO arises as a consequence of the activation of neuronal nitric oxide synthase (nNOS) located post-synaptic to the DA nerve terminal, in a subpopulation of striatal interneurons. Interestingly, it has been shown that rats with partial DA loss induced by high doses of METH do not exhibit further striatal DA loss when later re-exposed to a second neurotoxic regimen of METH. Thus, it appears that animals with partial striatal DA depletion are resistant to further METH-induced neurotoxicity. Preliminary evidence suggests that nNOS activation in striatal interneurons is increased 1 hour after a neurotoxic regimen of METH, but not in animals who received a prior neurotoxic regimen of METH (i.e. those animals with a partial DA depletion). Furthermore, it has been shown that partial DA loss is associated with altered function of the DA D1 receptor-containing striatonigral efferent neurons. In light of these observations, the overall hypothesis of this project is that DA-mediated activation of the nNOS- containing striatal interneurons is necessary for METH-induced neurotoxicity. Three specific aims are proposed to test this hypothesis by 1) determining whether inhibiting activation of nNOS-containing interneurons in the striatum blocks METH-induced neurotoxicity; 2) determining whether targeted knockdown of nNOS expression in the striatum blocks METH-induced neurotoxicity; and 3) determining whether prior DA depletion blocks subsequent METH-induced neurotoxicity. Completion of these experiments will provide novel insight into the role of DA signaling and nNOS-containing striatal interneurons in METH-induced neurotoxicity. Information gained as a result of these studies would allow for the development of targeted interventions to address the neurotoxic potential of METH.